Preserve keyword-only defaults in namespaced_function and alias_function

[eeshsaxena]

What does this PR do?

Fixes #69901.

salt.utils.functools.namespaced_function and alias_function both rebuild a function with types.FunctionType, passing argdefs=fn.__defaults__ but nothing for __kwdefaults__. The constructor has no parameter for keyword-only defaults, so the copy loses them. Calling the copy without those keyword arguments then raises TypeError even though the original had defaults:

from salt.utils.functools import namespaced_function

def foo(bar=None, *_, baz=None):
    pass

namespaced_function(foo, globals())()
# TypeError: foo() missing 1 required keyword-only argument: 'baz'

Setting __kwdefaults__ on the new function object in both helpers restores the defaults.

Tests

Added tests/pytests/unit/utils/test_functools.py covering both helpers: each asserts __kwdefaults__ is carried over and that the copy is callable without the keyword-only argument. Both assertions fail against the current code and pass with the change.

I could not run the full salt suite on my machine (Windows, no salt runtime), so I verified by executing the patched salt/utils/functools.py directly with the two salt-internal imports stubbed; both copies keep __kwdefaults__ and are callable. The new unit test is standard and should run cleanly in CI.

Changelog

Added changelog/69901.fixed.md.

[twangboy]

Please make this against the earliest branch where the bug exists. Most likely 3006.x

[eeshsaxena]

Retargeted to 3006.x and rebased. I confirmed the bug is present there too: namespaced_function/alias_function build the copy with types.FunctionType, which carries __defaults__ but not __kwdefaults__, so keyword-only defaults were dropped on that branch as well. The fix and unit tests now sit on top of 3006.x.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Preserve keyword-only defaults in namespaced_function and alias_function), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining salt!

[twangboy]

Looks like it's failing pre-commit and lint.

[eeshsaxena]

Thanks for the heads up. I'll run pre-commit locally, clear the lint failures, and push a clean commit.

[lkubb]

This is a duplicate of my PR btw #69902

[eeshsaxena]

Ah you're right, #69902 is older and covers the same ground. I'll close this in favour of yours. If the keyword-only defaults regression test here is useful feel free to lift it, otherwise no worries.